The carbon content of molten iron is an important variable in iron foundry production since the amount of carbon has an important effect on the mechanical and metallurgical properties of the final iron product.
Generally the carbon content of molten iron may be determined, prior to casting of the iron, by chemical analysis or by thermal analysis. Chemical analysis is generally too slow for production facilities and essentially all production facilities use thermal analysis for the determination of the carbon equivalent of molten iron.
In the past, thermal analysis has involved the determination of the carbon equivalent of molten hypoeutectic iron (which carbon equivalent may be defined as the total carbon percent plus onethird of the total silicon content plus one-third of the total phosphorous content) by generating a cooling curve for the molten iron sample and noting the liquidus thermal arrest temperature (which is the temperature at which austenite starts to precipitate in the sample). The carbon equivalent-cooling curve technique is described, for example, in an article "Carbon Equivalent in Sixty Seconds," Modern Casting Magazine, March, 1962, pages 37 to 39. As noted therein, the liquidus break for hypereutectic irons (those with carbon equivalents above 4.3 weight percent) is not clear enough so this method is limited to hypoeutectic irons.
A method of utilizing thermal analysis of this type for hypereutectic irons is disclosed in U.S. Pat. No. 3,546,921, which discloses treating the molten sample (as by coating the sample cup) with a carbide stabilizer (e.g., bismuth, boron, lead, magnesium, cerium or mischmetal) which retards primary graphite formation in the iron during solidification to insure an arrest at the liquidus temperature. The carbon equivalent of the hypereutetic iron can then be determined by comparison of the initial arrest temperature with a reference chart based on samples which have been chemically analyzed and the carbon equivalent determined. The amount of carbon and silicon in the cast iron determines the physical properties and microstructure of the iron. When the carbon equivalent is determined by thermal analysis, the silicon content is measured by wet or spectrographic analysis (or by calculation if the total carbon content is known). Thus, two analyses are generally required to determine the carbon and silicon content.
The use of a cooling curve computer for detecting the liquidus and solidus temperatures of molten iron samples and computing the carbon equivalent therefrom by means of an empirical equation is disclosed in U.S. Pat. No. 3,891,834. If desired, the computer may also be utilized to store a signal representing the eutectoid temperature which may then be utilized to determine the percent silicon by another empirical equation. The percent carbon can be determined from the computed carbon equivalent and percent silicon values (assuming phosphorous as zero).
A need remains for an accurate, rapid method for the determination of total percent carbon in a molten iron sample which does not require either an initial determination of the carbon equivalent or a computer and which is applicable to both hypoeutectic and hypereutectic irons.